The present invention relates generally to fabricating shaped metal objects from relatively thin, flat blanks and more specifically to forming winged mower blades from milled blade blanks.
Typical of previous methods of fabricating winged mower blades or similarly shaped articles, an annealed blade blank was inserted into a press and cold-formed therein. This method usually required the use of more expensive previously processed steel strip to form the blank and required frequency straightening of the formed blade after the step of pressing. The press used with a cold forming process must have a large tonnage capability. A method of fabrication is needed that would reduce the need to straighten the blade after forming, lessen the tonnage requirement of the press, and decrease the cost of the metal stock. Such a method would include the step of hot-forming a blank cut or milled from an unannealed piece of metal stock.
To properly heat-treat an unformed carbon steel blank and prepare it for the step of pressing, it is necessary to heat the blank to above its curie temperature. If a conventional furnace is used, the heating time is relatively long, and scale tends to build up on the blank as carbon is drawn out of the steel. Shrinkage of the blank results from the scale buid-up. The finished blade is less attractive and requires an additional cleaning step when scale is present. One method of heating, which is relatively fast and which additionally reduces the overall size of the equipment is inductive heating. The conductive metal to be heated is typically placed on a support within an inductor coil which carries current from an alternating current power source. A time-varying magnetic field is established which induces eddy currents in the metal workpiece according to Faraday's induction law, and heat results from the resistance of the material to the flow of the currents.
A problem, however, exists when heating by induction if the workpiece is relatively thin and is to be heated above the curie temperature of the component metal. As the workpiece approaches the curie temperature the permeability, or measure of flux produced in a material by the application of a magnetizing field of a given intensity, decreases resulting in a decrease in the skin effect which increases the depth of current penetration. Reduction in efficiency of transfer of energy from the inductor to the workpiece results as the permeability decreases. When the flux produced in a relatively thin workpiece reaches a certain point, there is no more material to support the amount of flux that can be delivered by the inductor. The energy that normally would heat the workpiece is lost, for example, in the form of heat build-up in the inductor.
Overall efficiency of the unit depends not only on induction heating power transfer efficiency, but also on the conversion efficiency of line power to induction heating power. Motor generators and electronic oscillators previously used have been costly to set up and operate, requiring frequent maintenance and having limited conversion efficiency.
Heat-treating and hot-forming of a blade requires quenching of the blade after it is formed. Several problems exist when conventional quenching is used. First, oil base quenches produce smoke, odor and danger of fire. Second, if the blade is not properly oriented when it is deposited in the quench solution, the differential cooling will cause warping and will increase the need of straightening of the blade after it is treated.